This invention relates generally to the production of shadow masks for color television tubes and particularly to an apparatus and method detecting bare metal prior to the acid etching of such masks.
A colored television tube includes a shadow mask which serves as the color selection electrode. The shadow mask includes a large number of apertures through which the electron beams pass to impact a phosphor of the proper color. Typically, in the manufacture of shadow masks the aperture pattern is photographically reproduced on a photoresistive material which covers both sides of the conductive metal strip from which the masks are made. The unexposed photoresistive material is then washed away leaving bare metal at the aperture locations. The shadow mask material is then exposed to acid which etches through the bare metal to produce the apertures. Typically, the shadow mask material is in the form of a long strip which is pulled through the etch line by a roller. The metal strip is coated on both sides and a large number of aperture patterns sequentially produced on the strip. A large number of masks are thus etched as the strip of material is pulled through a tank of etching acid. Problems frequently arise when the acid inert photoresistive material is not uniformly applied across both sides of the entire metal strip. The bare metal exposed by the voids in the photoresistive material is etched. When the voids occur on both sides of the strip they are completely etched through and the strip frequently is severed in the acid tank and the etch line must be shut down. On occasions when the void occurs on only one side of the strip only partial etching occurs. However, this also frequently results in a shutdown of the etch line because the partially etched metal is not strong enough to withstand the force required to pull the strip through the acid tank. In either event, the severing of the strip causes a shutdown of the etch line and requires entrance into the etch tank to retrieve and to repair the severed end.
The instant invention overcomes these problems by the provision of an apparatus which senses the bare metal exposed in the photoresist voids and generates a signal which can be used to cause the application of an acid resistant material to the metal strip to protect the strip during etching.